|Sep 16, 2013, 09:16 PM|
Lancer .95 Powered Advanced Flying Wing
This is a discussion about the design and build of a moderate size (.95 glow powered) flying wing I call the Lancer. The Lancer is a sport plane with the smoothness of a pattern plane, great maneuverability, and with excellent high angle of attack characteristics. It came about after much thought and with the help of a small prototype to test the idea. And, if you ask me, it looks really, really cool…(naturally).
Attached pictures show the final airplane. It has a 59 inch wingspan and weighs a little under 10 lbs with eflite retracts and OS 95AX engine.
|Sep 16, 2013, 09:21 PM|
I'll get to the Lancer shortly, but let me explain why I decided to develop this type airplane in the first place.
A few years ago I decided that there were few RC planes available that really caught my eye. I had stumbled upon the Simitar series created by Bill Evans and, since I had a fascination with flying wings, started to look seriously at building one. And, being something of a "hobby aerodynamicist" (whatever that is), I thought I might also bring something new to the table.
Many years ago, after getting married and having kids right away, I got away from RC for quite a few years - 27 to be exact. This is hard for me to believe even now! However, over the years I have built many (about a gazillion) hand launched gliders to experiment with many different types of aircraft, including conventional and unconventional planes. I also bought many books on aircraft design and lots and lots of books on airplanes in general. Upon studying all this brain fodder I started to look closely at some delta wing designs from the 50's up until today. I noticed in particular that many of the wings had the twist, camber, and thickness vary along the span, and that even the base airfoil varied as well from root to tip. Each span location had an airfoil with twist, camber, and thickness to match the local flow, particularly on swept wings and deltas. This allowed delta to perform much better, especially at maneuvering angles.
For example, look closely at the later versions of the Convair F-102 and F-106, and the Mirage III. Note the "conical camber" introduced along the leading edge. Contrary to what might be thought, this did not reduce speed, and in fact added significantly to the altitude, maneuverability, and especially handling of those fighters. Other good examples are the Concorde SST and the Eurofighter Typhoon. Look at a front view of these aircraft and note the highly sculpted wing.
I also have a book written by Alexander Lippisch about the flying wings he developed before and during WWII. His early attempts at tailless planes met with something less than success, with poor gliding (most of his early designs were for competition gliders), and even worse handling. What he did to solve these problems was similar to what came later for fighter type delta airplanes. He realized that the root and tip of the wing had to perform quite differently and that the airfoils needed to be tailored to those conditions. Unfortunately, he chose to use a very thick airfoil at the root (trying to maximize lift there) and a very thin airfoil at the tip with elevons incorporated only near the tip. This lead to the root never stalling and the tips stalling way early, with ensuing handling problems. He kept the airfoil distribution, but added slots in the leading edges to moderate the flow breakdown at the tips. This worked OK, but was short of what could be accomplished.
Another, but little known, designer that was also working on flying wing designs was Rudolf Gothert. He was developing his own flying wing fighters in parallel with Lippisch and the famous Horten brothers. He actually attempted to obtain contracts to build the Ho-229, but thought that the 229 was poorly designed for a large, jet-powered fighter. He started a series of designs that led to a fabulous tailless plane call the Gotha P.60. This was never built, but I'm convinced that its performance and handling would have caused serious problems for the allies had it come to fruition – even more so than the ME-262. It would have made the most out of the low thrust jet engines with high fuel consumption, and also been highly maneuverable at the same time. He did indeed varied the airfoil, twist, and camber along the span and added real vertical fins and rudders. My apologies to all of the Northrop flying wing and Horten brothers fans out there – but they REALLY needed vertical fin(s) and rudders! And Gothert did not try to make the fuselage totally integrated with the wing (as Northrop/Horten did) which would only make it ridiculously thick and bulky and draggy and prone to all kinds of flow separation.
So, with this and lots of other flying wing info in hand I started building balsa and foam gliders with all kinds of warped and twisted wings. I was able to build several of these that required no airfoil reflex or elevons at all and these would out glide darn near anything else with the same wing size and aspect ratio.
So after getting back into RC and building a number of airplanes, I decided that I would start with something like Bill Evan's Pole Star fuselage and come up with my own wing using some of what I thought I had learned. Using simple twist with a swept wing had been used for many years for flying wing gliders to balance flight, but still using one airfoil for the whole wing. I wanted to improve on this. Twist is great, but can add drag and tends to work well only at one airspeed. By using a small amount of twist (washout) along with a different airfoil at the tip, suited to the airflow conditions it would see, combined with conventional camber I thought I could balance flight and literally eliminate any need for a reflexed airfoil (or trimming up the elevons to obtain reflex). The idea was that the more cambered airfoil would have a steeper lift-curve slope and add the necessary stability not quite provided by the small twist angle. This provides the necessary positive pitching moment at zero angle of attack that decreases with increasing angle of attack (i.e. stable).
The issue with the above is that the swept wing will still tend to stall at the tips first, so I elected to do three things: 1) add a small amount of conical camber at the leading edge which makes the tips less sensitive to angle of attack, 2) increase the airfoil thickness at the tip and use a softer stalling profile, and 3) add wing fences to inhibit span-wise flow. I've used wing fences many times on even slightly swept wings with amazing results.
All this "stuff" seemed to gel in my mind and I decided to build a small (48" span) prototype to test these ideas. The outcome of that was a 0.55 powered plane I call the Dagger. The Pole Star had a simple fuselage design and I based my Dagger on something similar, but extended the fuselage aft enough to put a fin well behind the CG. I also put the fuel tank "on" the CG and used a Perry pump to feed the engine. Since the tank would be slightly higher than the engine I also employed a "Uni-flow" fuel tank system to eliminate the effective head.
The resulting wing for the Dagger has a lot in common with the Eurofighter wing. The root does have a small amount of negative camber in the middle, but the trailing edge is flush with horizontal so there is no real reflex providing “up” trim there. Essentially the nose of the airfoil is reflexed up a bit rather than the trailing edge. This shifts the lift forward a tad at low angles of attack, but then it shifts aft faster than normal resulting in a reduced lift curve slope. The tip has positive camber, washout, and more nose camber than the root. This results in the center of lift shifting inboard and forward on the wing a low angles, then aft and outboard and higher angles – so with a swept wing stability is assured and more powerful than the small 1.5 deg washout would provide alone. I did not want to overdo this, and I do like to fly fast as well, so all these ideas show up as only subtle effects on the wing airfoils, but it turns out a little goes a long way. Ideally the middle of the wing would have a different airfoil and washout as well, but I did not want to mess with a multi-section wing (at least not yet).
I built the Dagger and when I took it out for the first flight I was more nervous than I had ever been. One reason is that I also decided that all the "stuff" I incorporated into the wing would make it stable enough to use a small margin of stability (<9% MAC ahead of the neutral point), which is on the low side for a sport plane designed to maneuver aggressively. Gliders will sometimes use a stability margin even lower (down to 5%) but their speed range is small, they don't have a prop blasting back over the wing and they don't maneuver that hard. I note that the Simitar series use a stability margin of 15%, which is on the nose heavy side. I have built a couple of small RC flying wings many moons ago, and did use a ~15% CG location. They flew OK and would not stall easily, but the real reason is that they had a very low wing loading and the elevons just did not have enough power to stall the airplane with such a nose heavy condition.
And to make things more thrilling -- I was also guessing at control throws. But I do know a little about aerodynamics and calculated elevon moments vs stability moments and compared those calculations to my "conventional" airplanes. Not to brag too much - but I got the control throws spot on using this method. By the way, low rate on the Dagger is just 1/4 -5/16" up/down, and high rate just 3/8" - but enough to turn a really square corner.
I wanted to design a normal weight (23-27 oz/sq.ft), high powered, highly maneuverable airplane that could fly at really high angles of attack with no loss of control. And I almost got that with the Dagger -- almost. At high turn rates the Dagger would, well, roll and wobble for lack of a better word. Did not totally lose control, but it was not so smooth. Now remember I was flying at extremely high angles of attack with an instantaneous pitch rate - so the design was working really well as is -- I just wanted even more! Experimentation followed. First thing was to add the wing fences - and got much more lateral control, to the point that it would not roll off, but it still wobbled. The wobble was pretty much in the plane of the wing, and I suspected that flow separation at the wing-fuselage junction was affecting the vertical fin. OK, the Mirage 2000 fighter had the same problem and the cure was small fences (really small canards) on the fuselage in front of the wing. I thought, "I can do that", so on went small fences just in front and above the wing and -- magic, no more wobble and rock solid in tight turns. The only other little thing I noticed was the pitch rate was so high it was less precise that I wanted. This I thought, was because flying wings really have low pitch damping, leading to overshoot and "hunting" for the correct angle of attack. I thought (again) I can fix that -- with strakes aft of the wing to increase pitch damping. They also allowed me to move the CG even further aft.
After all that was done I had something that was marvelous - a flying wing that was as smooth and graceful as any pattern ship, as maneuverable as any lightly loaded airplane (if not more so), could fly at crazy angles of attack all day long with just a couple clicks of throttle, and turn at full speed on a very small radius. It is also very fast and out-climbed most anything other club members could throw against it. I flew the wings off that plane for two years, almost to the exclusion of all other planes as nothing else was as much fun. A number of club members flew the plane and their reaction was unanimous -- surprisingly smooth and maneuverable. One guy flew it on extra high rates and said it was like flying a mosquito! And that was the only shortcoming -- it was small and had no retracts. Time to scale this puppy up and add retractable landing gear...
|Sep 16, 2013, 09:26 PM|
Before I started on the Lancer design I had run some calculations on stability and control power vs wing sweep, and decided that a little more wing sweep would be a good thing. Unfortunately, the wing sweep, combined with the longer nose needed to fit retracts, gave me concern about balancing the new plane, especially with the .90 size engine I was planning to use. Luckily, OS came out with a .95AX that was the same case as their .75AX – and I had a .75AX on hand. So, I could build and test fly with the .75 and if all was well move up to the .95 with actually a slight decrease in weight and no real change in CG.
To help manage the balance I notched the front of the wing so that it would overlap the nosewheel and allow a more forward CG. I also extended the tail of the airplane further aft and planned to put both the receiver and retract batteries at the extreme rear. I intended on using the eflite 60-120 electric retracts.
As in the Dagger, I located the fuel tank near the CG, but not so far aft that it might cause a fuel draw problem even with the pump. It ended up just few inches forward of the CG which does not cause a significant weight shift with fuel burn.
It took a surprising amount of time (about a year) to finalize all the details to my liking and get plans drawn up to build from. The Lancer ended up with more refined airfoils than the Dagger, in particular having more camber at the tip which went along with the increased wing sweep.
I am fortunate in that I have access to the full version of Autocad and I built a detailed 3D model of the Lancer along with the 2D plans. Between the two I finally got the aerodynamics I wanted combined with a nice looking aircraft design, and it is a simple plane to build. As in the Dagger I used an off-the-shelf canopy and top deck, this one from the Sig King Kobra, to turn the simple box construction into something more aesthetic. I also kept the nose as slim as practical to keep it looking jet-like.
I’ve attached the plans to the next post. They are full size pdf prints, but you can print them smaller, say 11x17 and follow along with the build. If you want full size prints you can bring the files to Fedex/Kinkos (or any print shop) and have them printed full scale (“no scaling” option in the pdf print dialog). The actual page sizes are included in the filenames.
I also have airfoil data files (.cor) for this plane, as well as a dxf file for laser cutting of the plywood frames. If anyone decides to actually build this plane send a PM or email to me and I will forward the necessary files.
At this point in time I have about 20 flights or so on the Lancer, with the last 6-7 using the OS .95AX and 13x7 prop. Still breaking in the engine, but the Lancer’s handling and performance are everything I wanted. It is no super speed demon, but it’s not slow and if you enjoy smooth and graceful aerobatics combined with aggressive maneuverability, this may be the plane for you.
So, on with building the Lancer. I hope you enjoyed reading my notes above about how this came to be. It is certainly an unusual and attractive plane, and it flies with the best of the best.
|Sep 16, 2013, 09:36 PM|
Attach are pdf plans for the Lancer. This airplane is intended for experienced modelers, or those that could obtain help from a more experienced person.
If you choose to build this airplane, I strongly recommend you take the time to build accurately. This is true for any plane, and flying wings in particular work best when everything is properly aligned and true.
As mentioned previously, you'll need to save the plans, then have them printed at full scale. The actual page size is in the filename. When printing make sure that in the pdf print dialog that the scaling is set to "no scale". When I printed these at Fedex/Kinkos it took the worker a couple tries to get the prints to come out right. You can also print these at any page size if you just want to follow along. I recommend a minimum of 11x17 so you can see all of the details.
|Sep 19, 2013, 07:57 PM|
Some time ago someone said he was going to build an electric Dagger, but I have not heard if he ever did or not. The Dagger would probably be a good choice for electric power, but as I said, I don't really know much about this.
I will start loading build photos with discussion by the weekend.
|Sep 20, 2013, 09:55 AM|
Joined Oct 2001
There are some very powerful motor/battery/prop combos out there. Weight might not be an issue; could be a simple "drop-in". Someone would have to do their homework to find out.
I have quite a few electrics in the hangar but I love the smell of glow fuel too. The Dagger will definitely be glow.
|Sep 23, 2013, 08:43 AM|
Canada, BC, Kamloops
Joined Oct 2011
I really enjoyed and appreciate the well written design rationale explanations that help me relate to your goals for your wings and your clever solutions that resolved real life performance issues.
I built an very simple light weight foam board electric powered version of the Simitar "Deuce" which I just loved and will be following your thread on the Lancer.
I want to attempt to build an electric Lancer in a simple adaptation made of foam board in a 40" wingspan.
Thanks SO much for sharing your detailed plans.
|Sep 23, 2013, 03:49 PM|
Basic fuselage construction
OK, here we go...
Before you start to build, I strongly recommend you take the time to build accurately. This is true for any plane, and flying wings work best when everything is true.
Start by laying out the fuselage sides (see drawing sheet 4). I cut the fuselage outline from the plans and lay them right on the wood. Then I mark all locations and cut the wood using a long and straight metal ruler and a sharp x-acto. Add all of the doublers to the fuselage side balsa. I used Titebond 3 for gluing the doublers, placing books on the sheets to hold them flat. Remember to make a left and right side! See photos 1-4.
Add the ¾ angles to the sides. Sand a slight angle at the rear of the front piece (the break line edge) and glue (epoxy) the front ends onto the aft ends, raising the nose to the correct angle noted on the drawing. Photos 5-11.
Set the fuselage sides on the top view of the plans and pin solidly in place. Using a plastic triangle to ensure vertical, I tack glued frames 5 and 7 with thick CA, then flowed in thin CA to solidly glue in place (photos 13-15). Then glue frame 7a (horizontal) against frame 7 (photo 16-19). This forms the upper and lower battery compartments at the rear.
Laminate frames 4 and 4a together, then install at the correct location at the break line. A small clamp helps hold this in place while you glue (photo 20).
The next step is to laminate the firewall (frame 2) from the three 1/8 ply pieces, then glue in place at the front of the fuselage after carefully aligning. This requires some precision, but it is not difficult if you follow along with the photos (21-24). Use a clamp to gently hold the firewall in place, then align it with gentle taps. Use a triangle to place the top/front of the firewall exactly over the front edge line on the plans, making sure the bottom is exactly parallel with that line, but 3/64” (slightly less than 1/16”) behind it. This provides the ½ degree down-thrust. Do not try to obtain the required right-thrust on the firewall itself. I’ve found it is best to shoot for one angle at a time. I use 0.020 plastic spacers under the left side engine mounts to set the right thrust (more on this later).
Lastly, glue frame 3 into position (photo 25). You now have the basic structure of the Lancer complete and it should look like photos 26-29.
|Sep 24, 2013, 02:35 PM|
Canada, BC, Kamloops
Joined Oct 2011
Great build journal!
These detailed photos and build log are tremendous!
Thanks for sharing - this will really help all of us wishing to get in on the fun all of your efforts will make possible.
|Oct 02, 2013, 12:58 PM|
Finally got some time to add to this build. I am taking my time somewhat to think about how I ended up building this thing so I can pass along what I learned.
I did forget to mention earlier that the firewall should be predrilled for the engine mount and throttle pushrod. If you use the engine mount shown you can use the marking pattern on the plans, otherwise you will have to center and mark your own. Note that the engine is installed slightly to the plane's left of center to allow for right thrust (this puts the prop on center). Temporarily bolt the engine to the firewall using blind nuts (T-nuts) on the back side. Tighten the bolts to pull the blind nuts fully into the wood - but do not over tighten. I put some thin CA under the blind nut flanges to help hold them in place. Remove the engine mount before you glue the firewall into the fuselage. [Sorry, should have mentioned all this earlier].
You would nominally install the engine so that the engine's prop hub is 5-3/8" from the rear of the engine mount, giving about 1/16 clearance behind the spinner. Be careful however, as you need to check the spinner you plan to use to see if more is needed. For example, I used a DuBro plasitc spinner and the back plate is recessed about 1/8", so I needed to add this to the engine location for a total of 5-1/2" (photo 30). Locate the engine, then drill and tap the bolt holes. There isn't any way to get to nuts on the back side, so tapping the holes on the mounting beams is about the only way to do this. I find this works OK as long as you don't overtighten and strip the holes. Just FYI, I drilled and tapped another hole in front of the engine for a test. I then screwed in a bolt and tightened it until I stipped it out. I found out it is not hard to do, so be careful. But if you don't overtighten it seems to hold up well. Try using only 2 fingers on the short side of an allen wrench. I only use socket head screws for engine mounting.
Laminate the nosewheel mounting plate from 2 ea 1/8 ply pieces. Set the nosewheel on the plate, center it up leaving about 1/8" from the front of the plate, and drill pilot holes appropriate for the screw size. If you haven't already, glue the hardwood rails in first, then glue the nosewheel plate in place behind the firewall, aligning with the marks on the fuselage size. Note that this plate is set at a slight angle - higher on the rear (photos 31, 32).
Test fit your nose gear (photo 33) and screw it down, then remove it. If you choose to add the tailwheel (recommended) then cut the plywood, bend the wire, and glue only the pieces that form the groove in place, don't install the strut yet. Photo 34 shows my tailwheel strut on its frame, but not yet with its ply cover plate. Glue the tailwheel frame into place in the fuselage, lining it up with the bottom edge (photo 44).
Paint the firewall, front and back, and the nosewheel plate, with a fuel proofer (I use finishing epoxy), and also up to about one inch in front of the firewall. When dry install the engine mount onto the firewall (photo 35) before you start to close up the fuselage. This will be your last chance to tweak the engine installation, so ensure all is correct (photos 35 36).
Now add the tank support plate, using the tank itself to locate the plate (photo 37). Allow for some foam cushioning around the tank, but you need the plate to be recessed at least 1/8" inside the wing saddle. Whenever possible I install my tanks so that they are tilted back a few degrees. This helps ensure that your fuel pickup is wet on landing approach even if you ran the tank down a little far. In this plane I just put the tank in place, added 1/4 balsa spacer, then glued the tank plate in place. If you use a different tank, just take a little time to locat the plate and any other spacers you might need.
Now you are ready to start closing up the fuselage. Just cut and glue (photo 38). Leave the bottom forward of the wing uncovered for now.
At this point I stepped out and built the vertical fin and rudder (photos 39 and 40). It is a simple design mostly of 1/4 x 3/4 sticks covered with 1/16 sheeting, plus a 3/8 top cap. The rudder is made from two 3/8 balsa pieces (note the grain direction).
OK, back to the fuselage. Add the sheeting for the nosewheel area. Here I just left a hole large enough to get the nosewheel unit in and out, planning to close it up some later (photo 41).
Now you need to add the filler blocks (1/4 sheet balsa) in the nose area to allow carving this area round. I used what pieces I had left and fit them in place. I recommend titebond type glue as it is strong and yet easy to sand. See photos 42, 43, 45, 46.
Once you get the fuselage fully covered it's time to make the cut out for the engine. To do this I laid the engine in place on the fuselage side, then marked a cutout just big enough to get the engine in. I removed the carburetor to simplify this step (don't forget to protect the engine from dust). I also had to undercut the side sheets a bit to slide the engine in (photo 47, 48). Screw the engine in snuggly in place, but you should not need to tighten the bolts very much - just enough to seat it on the beams.
Sand the front face of the fuselage so that with the frame 1 (donut) in place it lines up with your spinner back plate and clears by about 1/16" or so. Just take your time here and get it well aligned. Using your spinner plate for final alignment, epoxy frame 1 in place. When done it should look like photo 51).
This completes the fuselage structure. Now go out and buy a whole heap of sandpaper.........
|Oct 02, 2013, 02:33 PM|
** Correction for engine mount **
Doing this real time without checking it over much will let me make mistakes, so bear with me, and read this whole thread before you attempt to build.
I forgot to mention that BEFORE YOU INSTALL THE ENGINE MOUNT onto the firewall you need to put a 0.020 shim under the left side of the mount. This will provide the necessary right thrust. I used .020 plastic sheet (1/4" x 2.5" strip) from my hobby shop, cut to fit arount the bolts.
|Oct 14, 2013, 11:51 AM|
At this point you need to make sure your elbow is in good condition as it is time to sand the fuselage into its final round shape. I kind of cheated a bit, using a coping saw to cut the corners off at a 45 deg angle (photo 52), cuting just down to the tip of the triangles inside. You can see in photo 53 some of the tools I used to do this. Although I did use my power tool a bit, I tend to avoid power tools as it is just too easy to slip and mess things up real quick. I just used knives, shavers, and lots and lots (and lots) of coarse sandpaper (80 grit) to get near the final shape, switching to 150 grit as I got close to final. From the frame 3 and back, you should sand down to expose about 1/4 to 5/16" of the corner triangles. Up front you need to start tapering towards the front ring. Don't overdo this. See photos 54-58 to see about how this should look - noting the underlying triangles and fillers that are exposed.
You have heard this a lot I'm sure, but if you want a nice finish (even if you are using iron on covering) you must prepare the undelying balsa. I final sand using 200 grit using my hands (not a block) to apply even pressure on the sandpaper around the curves. This takes a little practice, but taking your time makes up for a lack of practice.
Just work it all slowly and get the shape right and symetrical on all sides. I stop at 200 grit, but you can go on to 300 or 400 if you like. I do something unusual though, and it works best if you stop at 200 grit. I use paper towels for the final sanding. Yep, you heard right - paper towels. Rubbing hard and fast, the paper towel burnishes and smooths the surface really well. This allows me to press pretty hard without having to worry about taking too much off. It's amazing how quick even 400 sandpaper will go through balsa if you are not careful. Anyway, the idea is to burnish the wood and remove all the remaining fuzz.
If your fuselage looks like the photos then you did good and now one arm is bigger than the other (if is wasn't already).
Wings are next...
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